The present invention relates to the general field of gas turbines, and more particularly to controlling a starter-generator.
The field of application of the invention is that of gas turbines for engines for airplanes or helicopters, and also for auxiliary power units (APUs).
In a gas turbine, a certain number of pieces of equipment, or accessories, are driven by a mechanical transmission from mechanical power taken from a turbine shaft. This mechanical transmission, which is referred to as an accessory gearbox (AGB), comprises a set of gearwheels housed in a casing and mechanically coupled to accessories. These accesses comprise in particular various pumps for producing hydraulic energy, feeding fuel, and providing lubrication, and in certain applications one or more electric starter-generators (S/G). Through the AGB, the speed of the S/G is in particular proportional to the operating speed of the engine, with the coefficient of proportionality corresponding to the gear ratio of the line of gearing in the AGB.
When the gas turbine is in operation, the or each S/G operates as an electricity generator and produces a voltage that powers one or more electricity distribution centers for the aircraft and its engine(s).
When the gas turbine is stationary or rotating slowly, an S/G can operate as a starter by being powered from an external energy source in order to set the gas turbine into operation by rotating the turbine shaft to which the AGB is connected.
The optimum mechanical conditions for operation of an S/G are contradictory depending on whether it is operating as a starter or as a generator.
For given mechanical power, when an S/G is operating as a starter, it is desirable to give precedence to high speeds of rotation in order to minimize the torque of the S/G, while also providing sufficient torque to enable the engine to be started. In the absence of a change of gear ratio, having recourse to a ratio that gives precedence to high speeds in starter mode leads to a wide range of speed variation in generator mode, thereby increasing the frequency range of the S/G in a manner that is prejudicial to integrating it in the electricity network of the aircraft. In particular, the upper limit may be unacceptable (greater than 800 hertz (Hz), which is generally acceptable on certain commercial airplanes).
The gear ratio between the turbine shaft and the S/G is consequently selected so as to provide a usable compromise between operation of the S/G as a starter and as a generator. In certain commercial airplanes, it is generator mode that determines the gear ratio to the detriment of starter mode.
In order to solve this problem, the Applicant has made proposals in patent application number FR 09/54983 for an S/G control system in which a gearbox is interposed between the turbine shaft and the S/G, the gear ratio of said gearbox being changed as a function of the mode of operation of the S/G. Nevertheless, such a system presents the drawback of requiring a device to be added for controlling the change in the gear ratio of the gearbox.